The present invention relates to an energy cable having a crosslinked electrically insulating layer, and to a method for extracting crosslinking by-products therefrom.
Cables for transporting electric energy generally include at least one cable core. The cable core is usually formed by at least one conductor sequentially covered by an inner polymeric layer having semiconducting properties, an intermediate polymeric layer having electrically insulating properties, an outer polymeric layer having semiconducting properties. Cables for transporting medium or high voltage electric energy generally include at least one cable core surrounded by at least one screen layer, typically made of metal or of metal and polymeric material. The screen layer can be made in form of wires (braids), of a tape helically wound around the cable core or a sheet longitudinally wrapped around the cable core.
The polymeric layers surrounding the conductor are commonly made from a polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked, as disclosed, e.g., in WO 98/52197. The crosslinking step, carried out after extruding the polymeric material onto the conductor, gives the material satisfactory mechanical and electrical properties even under high temperatures both during conventional use and with current overload.
The crosslinking process of polyolefin materials, particularly polyethylene (XLPE), requires addition to the polymeric material of a crosslinking agent, usually an organic peroxide, and subsequent heating at a temperature to cause peroxide cleavage and reaction. By-products are formed mainly deriving from the decomposition of the organic peroxide. In the presence of a continuous electrical field, such by-products, being entrapped within the crosslinked material, cause an accumulation of space charges which may cause electrical discharges and eventually insulation piercing, particularly in direct current (DC) energy cables. For instance, dicumyl peroxide, the most common crosslinking agent used for cable insulation, forms methane (light by-product) and heavy by-products, mainly acetophenone and cumyl alcohol. Methane can be eliminated from the cable core with a short degassing process at a relatively low temperature (about 70° C.), while acetophenone and cumyl alcohol can be removed only by subjecting the cable core to a prolonged degassing process, at a temperature suitable to cause migration of the by-products (usually about 70° C.÷80° C.) and subsequent evaporation from the cable core. This degassing process must be performed for a long time (usually from 15 days to about 2 months, depending on the cable dimensions) and cannot be carried out continuously but only batchwise in large degassing devices which can host a given cable length. This increases to a large extent the time and production costs of DC energy cables.
In US 2010/0314022 a process is described for producing an insulated DC cable with an extruded polymer based electrical insulation system, which comprises the steps of: providing a polymer based insulation system comprising a compounded polymer composition, preferably a compounded polyethylene composition; optionally cross-linking the polymer composition; and finally exposing the polymer based insulation system to a heat treatment procedure while the outer surface of the polymer based insulation system is covered by a cover impermeable to at least one substance present in the polymer based insulation system in a non-homogenous distribution, thereby equalizing the concentration of the at least one substance in the polymer based insulation system. The at least one substance comprises typically cross linking by-products and various additives, which typically increase the material conductivity. Preferably a thin metallic foil or similar is wrapped around the roll of DC cable. Alternatively, the impermeable cover can be the metallic screen or the outer covering or sheath arranged outside the metallic screen. The overall effect of such a process is that of equalizing as much as possible the concentration of the crosslinking by-products within the insulating layer, which, however, are not removed from the cable core.
JP 64-024308 relates to a DC power cable provided with a space charge buffer layer placed between the inner semiconducting layer and the insulating layer or between the outer semiconducting layer and the insulating layer, the space charge buffer layer being formed by a copolymer of ethylene with an aromatic monomer, e.g. styrene, in an amount from 0.01 to 2 mol % per 1 mol of ethylene. Due to the resonance effect of the benzene ring of the aromatic monomer, the surrounding electron energy is absorbed and the formation of space charge is prevented, and in addition it is possible to improve the dielectric strength of the base polymer.
JP 02-253513 relates to a cross-linked polyethylene insulation cable that should prevent oxidative degradation caused by contact with oxygen and should enable continuous operation at high temperatures. As by-product of the organic peroxide, cumyl alcohol undergoes degradation to form α-methylstyrene and water. The degradation of cumyl alcohol is accelerated in the presence of oxygen. The moisture that is formed by the above degradation may cause appearance of voids and bow-tie trees with consequent degradation of the insulating material. To prevent such drawbacks, a plastic material containing an oxygen absorbent is arranged on the central part and the outer semiconducting layer of the conductor.
As oxygen absorbent, a deoxidizer may be used, such as a commercially available product known as Ageless by Mitsubishi Gas Chemical Co., which is formed by iron oxide/potassium chloride.